The present invention relates generally to image recording devices, more particularly to cameras and video cameras, and even more particularly to the focus of such devices.
In order to record clear images of objects at varying distances from an image recording device such as a photographic film still camera, a photographic film motion picture camera, an analog video camera, a digital still camera, a digital video camera, or the like the image projected onto the film or image sensor used for image capture must be in focus. A typical method of obtaining such focus is to vary the distance between the lens and the plane of the film or image sensor either manually or automatically in relation to the distance from the object to the lens.
However, less expensive cameras can be produced using a fixed lens system. In such cameras, the distance from the lens to the plane of exposure is fixed. Now, however, obtaining a clear image becomes more difficult, as the camera must be moved to a fixed distance from the object at which the image of the object is in focus. The in-focus object-to-lens distance is defined by the focal length of the lens system and the lens-to-plane-of-exposure distance, wherein the lens-to-plane-of-exposure distance is the distance between the lens of the image recording device and the plane of the film or image sensor used to record the image of the object. Among other means, the in-focus object-to-lens distance can be obtained and subsequently adjusted by either (1) guessing the distance, (2) using an optical range-finder, (3) forming two images on an image sensor from two lenses located a short distance apart, using a mirror to move one of the images until the two sets of signals produced by the two images on the sensor have maximum correlation, and then correlating the object-to-lens distance with the position of the mirror, (4) measuring the time taken for an ultrasonic wave pulse to reach the object and be reflected back to a detector on the camera, (5) measuring the amount of light or infra-red radiation reflected back from a scene by an emitted flash and correlating that measurement to the object-to-lens distance based on the average reflectance observed with scenes of average reflectance, and (6) viewing the image prior to exposure as in a single-lens reflex camera. All these methods are useful, but all have their disadvantages. Guessing is at best error prone, while the other techniques are expensive and time consuming as they may require additional equipment and several iterations of measurements with subsequent adjustment of the object-to-lens distance before making an exposure. In addition, false readings may occur in some situations depending upon the technique used. As an example, a heavily illuminated scene could indicate a false object-to-lens distance if that distance is set using the measurement of object reflected light from an emitted flash. Lens systems having a large range of object-to-lens distances in which the image of the object is in focus have also been used but are useful only when object-to-lens distances are large.
Thus, there is a need for an image exposure system that can easily, accurately, inexpensively, and automatically capture focused images using fixed lens systems.
In preferred embodiments, the present patent document describes method and apparatus for obtaining a focused image using an image recording device having a fixed lens-to-plane-of-exposure distance, wherein the lens-to-plane-of-exposure distance is the distance between the lens of the image recording device and the plane of the image recording medium which could be for example photographic film or an image sensor. The image recording system includes light beam sources directed such that when the projections of their light beams onto an object overlay or intersect each other, the object is positioned at the correct distance from the image recording system to project a focused image onto the image recording medium.
While applicable for use with various lens-to-plane-of-exposure distances, the lens-to-plane-of-exposure distance can be fixed in representative embodiments providing the ability to manufacture relatively inexpensive cameras used for document recording.
In alternative embodiments, additional pairs of light beam sources are added to the image recording system to provide for rotational alignment of the image recording system with the object.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.